Efficient Removal of Fluoride Using Polypyrrole-Modified Biochar Derived from Slow Pyrolysis of Pomelo Peel: Sorption Capacity and Mechanism

In this study, a novel adsorbent was developed by loading polypyrrole (PPy) onto biochar (BC) prepared by slow pyrolysis of pomelo peel. This PPy-grafted BC mainly exhibited anion-exchange behavior, which dramatically increased fluoride adsorption capacity. The effects of various factors (adsorbent dosage, pyrrole concentration, initial solution pH, co-existing anions and temperature) on the fluoride adsorption were investigated. The adsorbent was characterized by scanning electronic microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and automatic titration methods. Results showed that the optimized adsorbent (BP-0.1) performed excellently at pH 2.8–10.0, and possessed positive charge at pH < 8.6. The adsorption isotherm data were fitted well by the Langmuir model and the maximum adsorption capacity was 18.52 mg/g at 25 ± 2 °C. The adsorption kinetics followed the pseudo-second-order model and adsorption equilibrium was achieved at 24 h. Although both HCO 3 − and CO 3 2− had great influence on F − adsorption, there was no significant impact in the presence of Cl − , NO 3 − and SO 4 2− . Real groundwater study showed that 2.5 g/L of BP-0.1 could effectively reduce F − from 10.0 to 1.4 mg/L at pH 5.0. Thermodynamic parameters confirmed the spontaneous and endothermic nature of the adsorption process. Effect of pH, XPS analysis and the change of amount between Cl − and F − in the adsorption process revealed that ion exchange and electrostatic attraction were involved in the adsorption process and ion exchange was the main adsorption mechanism.